Impacts of Manifold Geometry and Flow Configuration on Shunt Current, Current Distributions, and Crossover in an Alkaline Electrolysis Stack

Abstract

Alkaline diaphragm water electrolysis (ADWE) devices have undergone widespread commercialization for about a century to date. Their high resistance relative to polymer electrolyte cells has motivated the employment of ion exchange membranes. However, recent advancements in the zero-gap configuration, in which porous electrodes are in contact with the porous ZrO2/polysulfone diaphragm, hold promise in extending the use of this inexpensive and durable technology by eliminating a portion of resistive losses.Desired improvements to the technology include reductions in electrode void fraction, crossover, and shunt current. Lower void fractions increase current density by allowing aqueous ions to both migrate and react at a greater portion of the electrode surface. The geometry investigated utilizes a design in which electrolyte feed is injected from the manifolds directly into the porous electrode without an intermediary flow field, which promotes product gas expulsion 1. Crossover is of particular concern because it puts a lower constraint on the practical thickness of the porous separator. Different modes of operation are investigated in this work to understand what provokes gas crossover locally, as previously done 1. Shunt current arises whenever an electrolyte solution is fed to a stack containing multiple electrodes contacted in series, inducing potential gradients in the electrolyte and leading to ion migration. This can lead to local potential gradients in the manifolds that allow undesired corrosion of the current collectors to occur.This three-dimensional (3-D) computational fluid dynamics model of an ADWE stack of five cells in series aims to gather details of the aforementioned problems, notably how they vary in 3-D space. Void fractions and crossover are compared between simulations of co- and counter-current operation. The locations of potential gradients that threaten the lifespan of the system are identified, and this information is used to suggest strategies to reduce or mitigate high void fraction, high local crossover, and high potential gradients in manifolds. Additionally, as this is a stack model, through-plane current distributions and notes on the fluid flow rates from cell to cell are provided.References J. S. Lopata, S-G. Kang, H-S. Cho, C-H. Kim, J. W. Weidner, S. Shimpalee, Electrochim. Acta, 390, 138802 (2021). Figure 1